When preparing polyvinyl chloride (PVC) for manufacturing finished articles, various process-aiding additives are incorporated into the PVC for lubrication, stabilization, shock-proofing, for facilitating processing in special types of machines i.e. extruders injectors etc., for water-proofing, for providing special characteristics, for solubilizing the polyvinyl chloride with other compounding resins, etc., it is useful to incorporate such additives, in the form of master batches. In master batches, such additives are admixed with small amounts of polyvinyl chloride in order to facilitate the uniform distribution of the additives ythrough the final bulk of the entire batch for processing.
Among known processes for the preparation of polyvinyl chloride-based master batches, the following two have been the most commonly used: (a) the granulation process, (b) the dry blend process.
In the granulation process (a), the additives are premixed with some of the resin base before or are injected together with some of the resin base through the mixing chambers of processing equipment. This mixture is then processed by formation of a gel either via a gellifying extruder or between heated rollers to form a semi-molten gelled mass. The mass is then cooled to form solid rods or sheets.
The solid products of the additives, in the gelled resin-base, are then sub-divided into granules which comprise the master batch for incorporation into the entire molding composition. The composition is then shaped by extrusion, injection or similar conventional molding or shaping operations into the final product.
The dry-blend process (b) utilizes the dry admixture of the additives with polyvinyl chloride at elevated but below gelling temperatures wherein the various ingredients of the additives are absorbed by the polyvinyl chloride by raising the temperature of the mixing master batch ingredients. The temperature of the master batch is raised in the range of about 80.degree.-150.degree. C., usually by external heating or also by internal heating resulting from the mechanical friction of the polyvinyl chloride particles and additive materials against each other during vigorous mixing. After the entire batch has been raised to sufficient temperature for the PVC to absorb the additives, the master batch is cooled to room temperature, 20.degree.-35.degree. C. The dry blend process produces dry powder or granules. Where economy is desired over the aforementioned granulation procedure (a), the dry blend method is utilized. This dry blend method can be extended to the point where the polyvinyl chloride for dilution comprises the entire amount of the total processing composition. When this is so, the dry product with absorbed additives is directly introduced into the forming apparatus hopper. When less then the entire batch of PVC is utilized, the dry powder master batch is mixed in a slow mixer such as a ribbon mixer supplied with heated internal surfaces to provide the resin base with the necessary calories for it to absorb the entire amount of additives. This heated absorbed mixture is then the master batch to be distributed through the entire amount of the final PVC composition. Rapid mixing can also raise the temperature of the PVC and additives to the required range.
The granulation process (a) and the dry blend process (b) indicated above are inconvenient, uneconomical and usually degrade the final quality of the product. This degradation results from the heating of the PVC powder and the granulation matter to the usual temperatures in the range 80.degree.-150.degree. C. This heating prematurely uses up the "fresh" characteristics of the vast mass of the PVC resin and its additives. The ultimate effects of this loss of "freshness" are the lowered resistance to impact and a too-rapid thermal degradation of the final shaped product.